There are a variety of solid adsorbents which have been useful in adsorption and catalysis including commonly known materials such as activated carbons, activated clays, silica gel, activated alumina, and crystalline molecular sieves. Of these adsorbents, crystalline molecular sieves such as silicoaluminophosphates, aluminophosphates and aluminosilicate zeolites have been particularly useful because of their uniform pore size.
In many instances it is desirable to have the solid adsorbent deposited on a substrate as a coating instead of being contained in particulate form as pellets, beads, or other particles. There are several reasons why solid adsorbent coatings have been used including for example, to improve the catalytic or adsorption properties of the solid adsorbent by improving the surface area to weight ratio, to reduce the amount of solid adsorbent required, to protect the underlying substrate material from a harmful environment, to achieve a particular strength or form, and, to perform the particular adsorptive or catalytic function over the entire coated surface of the substrate.
Not surprisingly, there have been a diversity of end uses for solid adsorbent coatings. Moreover, the methods for applying the coatings have been varied and somewhat dependent on the particular end use.
For instance, in the area of corrosion prevention, U.S. Pat. No. 3,788,880 discloses a method of inhibiting corrosion of metal surfaces wherein a metal surface, e.g. of brass, is coated with a precipitate of a corrosion-retarding inorganic salt, e.g., a calcite from water, i.e., preferably ordinary utility water containing the salt and under the electrochemical effect of local polarity differences along the metal body, where upon the corrosion-inhibiting layer is used to bond, by ion-exchange interaction, an ion-exchange substance to the surface. Together, the corrosion-inhibiting and ion-exchange substances, i.e., a natural or synthetic zeolite, form a three-dimensional stable coating which is adherent and prevents localized corrosion.
Moreover, numerous lacquer or pigment type coatings have also been used in corrosion prevention, some of which contain zeolites. For example, U.S. Pat. No. 4,758,281 discloses a corrosion inhibiting pigment which comprises particulate zeolite that can be used in protective coatings for metals, e.g. paints, varnishes, lacquer, water-borne coatings, etc.
Also related to corrosion prevention, U.S. Pat. No. 4,610,700 discloses absorbent compositions useful in retarding corrosion in mufflers and sets forth at col. 5, line 10 to line 18, that:
"Coatings of tubes, bulkheads and/or internal surface with adsorbent is also feasible. Simulated coatings can be accomplished with an adsorbent loaded material or heat-resistant tape. Actual slip coatings made of silica-rich slurries of adsorbent powder can also be used to coat surfaces. Such a slurry can be used to dip, spray or otherwise cover any surface. The coating is hardened by heating the part to about 200.degree. C. either during production or on the vehicle." PA1 "In one aspect this invention provides a process for the preparation of a zeolite layer upon a substrate which does not have the same crystalline structure as the zeolite, in which process the substrate is contacted with a reaction mixture for forming a zeolite and the reaction mixture is heated to bring about zeolite formation, in which the substrate is tumbled within the reaction mixture during heating so as to cause zeolite formation preferentially as a layer on the surface of the substrate. PA1 The invention enables the zeolite to be formed as a layer on the substrate in preference to it being formed as pure zeolite particles within the body of the reaction mixture. It is believed that the mixing technique employed results in the preferential formation of nucleation centers upon the substrate surface rather than within the reaction mixture. Moreover, the product of the process of the invention comprises discrete substrate bodies coated with zeolite rather than agglomerates of separate zeolite particles with substrate bodies." PA1 "The slip for the first layer . . . is then applied on the metal substrate and dried at a temperature not exceeding 350.degree. C. to form the first coating. PA1 Next, a second slip comprising a frit and an oxidizing catalyst for the second layer is applied on the dried first coating. The application of the second slip is ordinarily made by spraying. The second slip coating is also dried, and then both the first and the second coatings are fired at the same time. PA1 For the metal substrate of the mild steel sheet for enamel, the firing is made at a temperature between 780.degree. C. and 820.degree. C. for 3 to 5 minutes. For the metal substrate of the aluminized steel sheet, the firing is made at a temperature between 520.degree. C. and 600.degree. C. for 3 to 5 minutes."
Solid adsorbent coatings have also been used as catalysts in chemical processes. For example, zeolite coated substrates intended for use in fluidized bed reactors have been prepared by a process wherein the zeolite itself is crystallized on the substrate in a reaction mixture. U.S. Pat. No. 4,578,372 discloses at col. 3, lines 5 to line 23 that:
Some self-cleaning cooking ovens utilize zeolite catalyst layers coated on the internal surfaces of the oven to catalyze the cleaning action. U.S. Pat. No. 4,460,630 relates to a method of forming porcelain enamels on aluminized steel wherein a zeolite-coating porcelain enamel layer is applied to an underlying ground coating of porcelain enamel on the aluminized steel. The abovementioned patent sets forth a coating method at col. 5, line 38 to line 53 wherein;
The term "slip" is generally used in porcelain enamel engineering to mean a slurry comprising a frit which consists of glass compositions that soften upon firing to form the porcelain enamel layer, and a binder as well as other additives.
Despite the diversity of coating methods and end uses known to exist, new methods are sought which can be used to coat aluminum substrates with solid adsorbent without the use of adsorbent formation reactions, frits and enamels, paints, varnishes and the like, in order to provide adsorbent-substrate composites that have physical and performance properties suitable for general use.
Some thermodynamic processes for cooling and heating by adsorption of a refrigerating fluid on a solid adsorbent use zeolite, and other sorption materials such as activated carbon and silica gel. In these processes, the thermal energy from adsorbing zeolite in one place is used to heat desorbing zeolite located in another place. U.S. Pat. No. 4,138,850 relates to a system for such solar heat utilization employing a solid zeolite adsorbent mixed with a binder, pressed, and sintered into divider panels and hermetically sealed in containers. U.S. Pat. No. 4,637,218 relates to systems for a heat pump using zeolite as an adsorbent wherein the zeolite is prepared by slicing natural zeolite rock with a carbide saw, or by pressing slightly-wetted, powdered zeolite into bricks. The bricks used in U.S. Pat. No. 4,637,218 are preferably not more than 10 mm in thickness.
U.S. Pat. No. 4,548,046 relates to an apparatus for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent. The operations employ a plurality of tubes provided with parallel radial fins filled or covered with solid adsorbent such as Zeolite 13X located on the outside of the tubes.
The thermodynamic aspects of developing a zeolite-water adsorption refrigeration unit are well known. An article entitled, "Thermodynamic Analysis of a Solar Zeolite Refrigeration System," by S. Chang and J. A. Roux, which appeared in the Journal of Solar Energy Engineering, August 1985, Volume 107, pages 189-194 provides a discussion of the main parameters, including adsorber properties. The above mentioned patents and articles are incorporated herein by reference.
Prior methods of using zeolite adsorbents in devices for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent have been inefficient and difficult to prepare. Those methods of preparation included cutting natural rock into thin bricks and mounting these bricks on to heat exchange surfaces or casting powdered zeolites and mixtures thereof with clays into panels or slabs for direct contact with fluids. Methods are sought to improve the operating efficiency of these devices, and to improve the way in which the solid zeolite adsorbent is employed in these devices.